Orthogonal Frequency Domain Multiplexing (OFDM) is a technique known in the art. In general, OFDM involves up-converting two or more low-rate data signals to respective sub-bands of a high bandwidth transmission channel. FIG. 1 illustrates an OFDM transmitter for implementing this technique in an optical communications network, which is known from Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF; Jansen et al., Journal of Lightwave Technology, Vol 26, No. 1, pp. 6-16, Jan. 1, 2008. As may be seen in FIG. 1, the transmitter comprises a pair of parallel transmission paths 2, each of which generates a respective sub-band (identified herein as sub-bands A and B) signal. Each path 2 comprises a digital signal processor 4 which generates parallel In-phase (I) and Quadrature (Q) sample streams based on a respective sub-band data signal (DT(A) and DT(B)) to be transmitted. Each sample stream is processed by a respective Digital to analog (D/A) converter 6, low-pass filter 8, and an amplifier 10 to produce an analog signal that is supplied to an input of an IQ-mixer 12. The IQ mixer 12 operates to mix the analog I and Q signals with a mixing signal having a desired frequency (f1, f2) to yield a pair of Intermediate Frequency (IF) signals 14, which are then combined to produce a single analog signal 16 containing spectral components of both of the analog I and Q signals. The combined signal output from the IQ-mixer is then Low-Pass filtered (at 14) to generate an analog sub-band signal 16 having a spectrum centered at a desired IF. An analog summation block 18 combines the two sub-band signals to yield a single drive signal 20 that is supplied to an optical modulator 22 (such as a Mach-Zehnder modulator). The optical modulator 22 is responsive to the drive signal 20 to modulate a continuous wave (CW) carrier light from a transmitter Local Oscillator (Tx LO) 24 such as a laser to produce a modulated optical signal 26 for transmission through the communications system to a receiver.
Suitable selection of the mixing signal frequencies (f1 and f2) results in frequency-domain separation between the sub-bands in the modulated optical signal. In the example of FIG. 1, in path A the mixing signal frequency (f1) is zero, so that the analog sub-band signal 16 generated by path A is a baseband signal having a bandwidth fs corresponding to that of the sub-band data signal DT(A). On the other hand, in path B the mixing signal frequency (12) is selected to be f2≧2 fs, so that the analog sub-band signal generated by path B has a spectrum centered at a frequency of ±f2 and having a bandwidth fs corresponding to that of the sub-band data signal DT(B). With this arrangement, the optical signal output from the optical modulator 22 has a spectrum 28 in which frequency components of each sub-band lay in respective different spectral regions of the optical signal. Consequently, sub-bands A and B can be readily separated and processed in a receiver, using known techniques, to generate recovered sub-band data signals DR(A) and DR(B) corresponding to the transmitted sub-band data signals DT(A) and DT(B).
A limitation of the arrangement of FIG. 1 is that the IQ-mixer 12 in each path combines the I and Q components into a single analog sub-band signal 16. This means that the optical modulator is restricted to modulating only the Real (or In-phase) component of the optical carrier light. No significant modulation of the Imaginary (or Quadrature) component of the optical carrier light is possible. However, as data rates increase, the tolerance for phase noise and non-linear impairments such as self-phase modulation and cross-phase modulation decreases. In order to maintain adequate noise margin for high data-rate long-haul optical transmission in practical networks, independent modulation of both of the Real and Imaginary components of the optical carrier light is important.
FIG. 2 shows an OFDM transmitter in which each path 2 is used to drive a respective optical modulator 22 to generate a corresponding optical sub-band signal 30. The optical sub-band signals are then optically combined to produce a modulated optical signal 32 for transmission. In the transmitter of FIG. 2, each path 2 outputs I and Q (or, equivalently, Phase and Amplitude) analog sub-band drive signals, which means that the Real and Imaginary components of the optical carrier light can be independently modulated. In order to provide spectral separation between the two optical sub-band signals, each modulator 22 receives a CW carrier light from a respective transmitter Local Oscillator (Tx LO) such as a laser. A controller 34 may then be provided to control the frequency difference Δf between the two sub-band carrier lights, so that the combined optical signal has a spectrum 36 in which frequency components of each sub-band lay in respective different spectral regions.
As noted above, because the I and Q components of each sub-band are available for driving the respective sub-band modulator, it is possible to independently modulate I and Q components (or Phase and Amplitude) of the sub-band optical carriers. As such, the OFDM transmitter of FIG. 2 is capable of higher performance than that of FIG. 1. However, this performance improvement is obtained by duplicating the electro-optical components (primarily the lasers 24 and the optical modulators 22), which significantly increases the cost of the transmitter. In addition, since two lasers are used, differential phase noise and line width of each laser can also impair performance.
What is needed is a cost-effective OFDM transmitter in which I and Q components (or Phase and Amplitude) of the optical carrier can be independently modulated.